Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
As long as the house has been your mian residence throughout your period of ownership, then you are correct in thinking there will be no capital gains - but if your siblings haled a share of the house too - then they will have capital gains on their share.
Then all of you will have capital gains on the land - as none of you enjoyed it as part of the home.garden - and as you used your share of the land purely for business, this is whay your charge arises.
The gain for each of you will be a third of the sale value less a third of the probate value - which forms the intial gain, From this initial gain you can deduct a third of the costs to sell (so legal fees and any estate agent fees) then the amount left has for each of you the first £11,000 exempt, then any remining gain liable to capital gains tax.
Capital gains rates are 18%, 28% or a mix of both and this is determined by what unused basic rate band you have after your normal annual income has been taken into account.
If your annual income is in excess of £41865 - then all of the remining gain will be liable to 28% as there is no unused basic rate band, whereas if your annual income was £30,000 then this would leave an unused basic rate band of £11865 , so the first £11865 of the gain would be libal to 18% and any remining gain at 28%
You should each alert HMRC of the sale, and you will just add the capital gain posityion into your next self assessment tax return.
Let me know if you wish me to expand on any of the information supplied.